Hey saydee...welcome..always happy to have another trifuler....
If you search the threads for wetsuits you'll find this same thread pops up several times a year...
I use and love the full. More warmth and because I sink like a stone the extra flotation helps...Have tried a sleaveless and just felt...partialy clothed! Some folks will rent a suit for their first few races to see which they prefer...
Make sure you take the time to fill out your profile so we know more about you...helps with advice...

I have a longjohn because it was cheaper and I liked the thought of having my shoulders free of friction or resistance to movement. Having said that the research clearly shows a full suit is more buoyant and therefore faster, perhaps on the order of 2 seconds per 100 meters, a significant time difference for the Olympic and longer swims.

Think about it. All the pros wear full suits.

BUT, the full has to fit properly, so you need to find someone that knows what they're doing to get you the right fit.

BTW, Anton, appreciate your helpful responses to these questions.